Atomic structure of a Na+- and K+-conducting channel

Ning Shi, Sheng Ye, Amer Alam, Liping Chen, Youxing Jiang

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Abstract

Ion selectivity is one of the basic properties that define an ion channel. Most tetrameric cation channels, which include the K+, Ca 2+, Na+ and cyclic nucleotide-gated channels, probably share a similar overall architecture in their ion-conduction pore, but the structural details that determine ion selection are different. Although K + channel selectivity has been well studied from a structural perspective1,2, little is known about the structure of other cation channels. Here we present crystal structures of the NaK channel from Bacillus cereus, a non-selective tetrameric cation channel, in its Na+- and K+-bound states at 2.4 Å and 2.8 Å resolution, respectively. The NaK channel shares high sequence homology and a similar overall structure with the bacterial KcsA K+ channel, but its selectivity filter adopts a different architecture. Unlike a K+ channel selectivity filter, which contains four equivalent K+-binding sites, the selectivity filter of the NaK channel preserves the two cation-binding sites equivalent to sites 3 and 4 of a K+ channel, whereas the region corresponding to sites 1 and 2 of a K+ channel becomes a vestibule in which ions can diffuse but not bind specifically. Functional analysis using an 86Rb flux assay shows that the NaK channel can conduct both Na+ and K+ ions. We conclude that the sequence of the NaK selectivity filter resembles that of a cyclic nucleotidegated channel and its structure may represent that of a cyclic nucleotide-gated channel pore.

Original languageEnglish (US)
Pages (from-to)570-574
Number of pages5
JournalNature
Volume440
Issue number7083
DOIs
Publication statusPublished - Mar 23 2006

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